# How to drive a small 8 Ω load given a CE-CE amplifier?

I want to amplify a 25 mVpp input signal with a source resistance of 1 kΩ to anywhere from 2.5 Vpp to 3 Vpp to drive an 8 Ω load (though at this point I am willing to sacrifice gain to just drive my load).

I successfully designed a CE-CE-Darlington pair amplifier configuration. The gain is what I want it to be with the exception of the issue of power dissipation across the secondary transistor. It exceeds the transistor's (2n9304 and 2n2222a) ratings and makes them release the magic smoke.

The circuit I have designed is below:

It might be odd but all I have are NPN transistors, specifically the ones I listed, so I can't use PNP BJTs though I wish I could.

Can I drive an 8 Ω load given my component limitations? I have managed to procure from an old bin some 2 W resistors and work around the resistors popping, but I am stuck at the secondary Darlington pair; I am not sure how to solve the issue. Any tips, tricks, or criticism would be appreciated.

• You can use a class-A output stage that is built entirely from NPN BJTs. Driving that also entirely from NPN will require some crafted thinking. But it's all doable.
– jonk
Nov 18, 2022 at 4:48
• An output transformer 10:1 would help to reduce the power dissipation in the output stage.
– Jens
Nov 18, 2022 at 5:02
• Use a lower supply voltage, perhaps 12 V or maybe lower with suitable base-biasing for each of the 3 steges.
– user173271
Nov 18, 2022 at 8:40
• The requirements to drive an 8 ohm speaker require design specs for current gain , impedance gain = Rin/8, distortion % and efficiency (opt.) If you would like my solution, pls include specs. Dec 14, 2022 at 16:47

You want almost 0.5V DC across $$\R_{bias}\$$ for silicon NPN transistors....$$\R_{bias}\$$ would be increased from 100 ohms to 270 ohms or perhaps 220 ohms is safer.